1. Field of the Invention
The present invention is directed to an improved in-situ retorting method wherein cocurrent and countercurrent techniques are combined in such a way as to retort the shale in a nonoxidizing atmosphere, achieve some in situ cracking which upgrades the quality of the recovered oils, and also improve the thermal efficiencies in the entire recovery system by recovery of most of the sensible heat from the retorted shales.
2. Description of the Prior Art
Two in-situ combustion processes have been proposed for recovering hydrocarbons from oil shale, the forward or cocurrent process and the reverse or countercurrent process. In cocurrent processes such as that described in U.S. Pat. No. 2,780,449 to Pelzer, the combustion front moves cocurrently, i.e. in the same direction as the oxidant flow. Cocurrent processes conserve heat energy because the injected air stream is preheated as it passes over the burned-out zone cooling the retorted shale and thereby recovering the sensible heat in an effective countercurrent heat exchange process. The preheated oxidant stream combines with the fuel left on the shale, i.e. carbonaceous residue on the shale and oil not yet completely educed from within the particles to provide the heat energy required to preheat the shale and free the shale oil in advance of the combustion zone. Shale oil, hydrocarbon vapors and products of combustion are driven ahead of the heat front and cool as they pass over the shale.
A disadvantage of the cocurrent process is that the combustion front will not move until all of the fuel at the front has been consumed. Therefore, it is desirable to supply only the proper amount of oxygen to burn the carbonaceous residue in the path of the advancing front while not burning recoverable hydrocarbon. The retorting rate is limited and is grossly influenced by the particle size of the rubblized shale. Furthermore, the high pour point waxy oils which such processes tend to produce cool to the ambient formation temperature in advance of the heat front fluid and can reduce gas permeability substantially.
To overcome these disadvantages, the reverse or countercurrent process has been proposed wherein the advancing combustion front (or oxidation zone) moves in a direction countercurrent to that of the injected air stream. As pointed out in Chemical Engineering Process Symposium Series, Volume 61, Number 54 (1965) by Allred and Nielson, this process is characterized by little or no preheating of the injected air stream, and the heat is retained behind the advancing combustion front. On the other hand, the process is such that the combustion zone movement is dependent primarily on the upstream heat transfer and not the total fuel available. Large particles of oil shale can, therefore, be effectively processed since they soak in a hot oxidant depleted gas stream at retorting temperature behind the combustion zone. Naturally, the retorted oil shale remains at or near retorting temperatures when the combustion front passes it. For this reason, the countercurrent combustion process does not make the most efficient use of the available heat. This process also tends to consume light hydrocarbons distilled from the oil shale rather than the carbonaceous residue on the shale. However, as is pointed out in U.S. Pat. No. 2,793,696 to Morse, elevated temperatures behind the advancing countercurrent heat front do provide natural cracking conditions in the burned-out zone through which the hydrocarbon vapors must pass. In the production of shale oil these natural cracking conditions produce hydrocarbons which are already visbroken and partially upgraded into lighter fractions. Because of elevated formation temperatures no condensation or subsequent loss of gas permeability takes place.
The present invention combines cocurrent and countercurrent techniques to retort the shale in a nonoxidizing atmosphere, achieve some in-situ cracking which upgrades the quality of the recovered oils, and improve thermal efficiency by recovering most of the sensible heat from the retorted shale.